Description of the Prior Art
In presently known data processing systems, recording of digital data is accomplished by registering binary data bits or pulses on magnetic media, such as magnetic disks. The digital data is encoded with clock or timing signals that establish windows or periods during which transitions or flux reversals representing data occur. In systems that process high density data, the bits are closely packed on the magnetic disk and are subject to interference effects and undesirable peak shift during the recording process. The write pulses may shift in phase and frequency depending upon the pattern of the coded binary bits. As a result, data readout may not be accurate if bit shift is sufficient to adversely affect the phase and frequency of the data signal being recorded.
To compensate for peak shift, write precompensation has been used during the recording mode to shift the write pulses, which in effect shifts the position or phase of the data bits, as they are being recorded on the magnetic disk. The precompensation causes selected bits to shift position with reference to the clock or timing signal so that the selected bits are recorded early or late relative to the center of the timing window. In this way, the anticipated shift of the write pulse, which occurs as a result of crowding of high density data coded in certain bit patterns, is effectively compensated and cancelled by the amount of shift provided by precompensation as the write pulses are being registered on the magnetic medium. In disk drive systems that use a constant transfer rate, it is known that the flux change spacing is greater at the inner radius than the outer radius of the disk surface. Designers of disk drive systems have accordingly changed the transfer rate radially across the disk in order to maintain a constant flux change spacing so that a greater transfer rate is obtained at the outer radius than at the inner radius. Therefore, for the purpose of data processing, concentric data tracks on the surface of the disk are arbitrarily divided into radial zones, typically three zones for example. By virtue of this approach of maintaining a constant flux change spacing, storage capacity of the disk surface is maximized.
Bit shift will differ for different transfer rates and track zones because changing the transfer rate implies recording at different radial locations on the disk surface. The physics associated with the recording process is different for different radial locations on the disk surface. One example is that the tangential velocity of the head-to-disk interface is greatest at the outermost or largest radial track.
To solve the problem of bit shift, different approaches to write precompensation have been provided. The magnitude of write precompensation in a track zone recording system is generally controlled by switching in different currents, voltages or resistances. Generally a voltage controlled oscillator (VCO) is used to control the operating frequency of the recording system. The VCO, which includes a capacitive element, is controlled to provide a constant frequency output signal. The VCO must be precisely controlled and the VCO capacitor needs to be matched to other capacitive elements and components of the write frequency precompensation circuit. It would be highly desirable to provide a precompensation circuit and associated circuitry on an integral semiconductor chip on which an integrated circuit is formed.